Micromachines (also called micromechanical devices or microelectromechanical devices) are small (micron scale) machines which promise to miniaturize instrumentation in the same way microelectronics have miniaturized electronic circuits. Micromachines include a variety of devices such as motors and gear trains analogous to conventional macroscale machinery. As is the case with macroscale machines, it is desirable to lubricate micromachines. Unfortunately, there are a number of problems associated with the lubrication of micromachines.
One problem with the lubrication of micromachines is that they have smooth surfaces and large surface-area-to-volume ratios. As a result, the intrinsic adhesion forces at contacting surfaces have greater significance than in macroscale machinery. Adhesion between moving and stationary parts, in excess of the small motive forces applied by micromachines, is a commonly encountered problem.
One function of a lubricant is to reduce such adhesion forces by serving as a low adhesion mediating layer, thereby preventing direct contact between higher adhesion mechanical parts. The lower the adhesion, the lower the friction and wear of the mechanical parts.
A fluid lubricant is typically used in macroscale machines to insure that fresh lubricant flows back over areas where interacting surfaces scrape the protective lubricant film. Unfortunately, conventional fluid lubricants introduce undesirable energy loss mechanisms of their own through fluid capillarity and viscosity. Thus, it would be highly desirable to develop a lubricant without the concomitant shortcomings of capillarity and viscosity associated with fluid lubricants.
The corrosive chemicals and high temperature processing involved in micromachine fabrication precludes lubricant application prior to the end of the fabrication process. At this terminal fabrication stage, the micromachine is fully and irreversibly assembled. The surfaces that must be lubricated are often hidden in the interior of the machine, and are perhaps already in physical contact with opposing surfaces. Consequently, the application of a lubricant must render full but not excessive coverage of lubricant on these hidden and contacting surfaces.
The terminal stage of micromachine fabrication processing involves removal of sacrificial layers encasing the functional parts of the micromachine. In one large group of micromachines, the functional parts are made of polycrystalline silicon (polysilicon) that is encased in silicon oxide sacrificial material. The silicon oxide is removed in a release etch using aqueous hydrofluoric (HF) acid. Following this etching, a micromachine is rinsed to remove leftover HF acid. Afterwards, it is dried.
In the prior art, a micromachine is lubricated after the drying step. The problem with this approach is that adhesion between the parts of the micromachine is more likely at this juncture. Thus, it would be highly desirable if the aqueous release etch steps could be combined with lubricant application steps. In such a process, the micromachine would be continuously immersed in liquids until the end of the process. As a result, a single drying step would be involved and the low-adhesion lubricant layer would already be in place when drying was completed.
Micromachines are often powered electrically. Electrical currents can leak across the surfaces of what are nominally insulating materials such as silicon oxide and silicon nitride. Leakage current results in energy loss. Electrical passivation of these surfaces to reduce this leakage is therefore a desirable function of the lubricant layer.